Pre-release: Add user profile update flow

[wauputr4]

Goal

Let signed-in users update their basic profile before release, instead of only viewing /api/v1/auth/me and the account page.

Scope

• Add authenticated profile update API for supported user fields, at minimum display name.
• Add account-page form and success/error states for profile updates.
• Preserve session validity after profile changes.
• Validate empty/invalid input and return stable error codes.
• Add backend and SvelteKit checks for the happy path and validation failure.

Definition of done

• A registered user can log in, update their profile, refresh, and see persisted values.
• Invalid updates show clear Indonesian and English feedback.
• go test ./..., npm run check --prefix web, and npm run build --prefix web pass.